Carbon Materials Comprising Carbon Nanotubes and Methods of Making Carbon Nanotubes

ABSTRACT

The present invention relates to carbon materials comprising carbon nanotubes, powders comprising carbon nanotubes and methods of making carbon nanotubes. In the methods of the present invention, the size and/or formation of floating catalyst particles is closely controlled. The resulting carbon nanotubes typically exhibit armchair chirality and typically have metallic properties. The carbon nanotubes produced by this method readily form bulk materials, which typically have a conductivity of at least 0.7×106 Sm−1 in at least one direction. The invention has particular application to the manufacture of components such as electrical conductors. Suitable electrical conductors include wires (e.g. for electrical motors) and cables (e.g. for transmitting electrical power).

BACKGROUND TO THE INVENTION Field of the Invention

The present invention relates to carbon materials comprising carbonnanotubes, powders comprising carbon nanotubes, and methods of makingcarbon nanotubes. The present invention has particular, but notexclusive, application to the manufacture of components such aselectrical conductors. Suitable electrical conductors include wires(e.g. for electrical motors) and cables (e.g. for transmittingelectrical power).

Related Art

Carbon nanotubes are allotropes of carbon, which are tubular andtypically have a diameter in the nanometre range. The carbon atoms of acarbon nanotube are each covalently bonded to three other carbon atoms,to create a “hexagonal” lattice which forms a wall of the tube.Accordingly, a carbon nanotube can be thought of as a “rolled” graphenesheet. Single-walled carbon nanotubes have a single layer of carbonatoms.

Double- and multi-walled carbon nanotubes have two or more layers ofcarbon atoms, respectively.

The chirality of carbon nanotubes can vary, depending on the orientationof the hexagonal lattice of the notional graphene sheet with respect tothe tube axis. Carbon nanotube chirality will be well understood by aperson skilled in the art. For example, carbon nanotubes may havearmchair chirality or zigzag chirality. Carbon nanotubes with achirality intermediate an armchair and a zigzag chirality are generallyreferred to as chiral carbon nanotubes.

Without wishing to be bound by theory, it is believed that allsingle-walled armchair carbon nanotubes are electrically conductive,regardless of their diameter (i.e. it is believed that all single-walledarmchair carbon nanotubes are metallic). Zigzag and chiral carbonnanotubes may be metallic or semiconducting.

(Nanotube chirality and properties, such as metallic and semiconductingproperties, are explained in detail in Reference 4, which isincorporated herein by reference in its entirety.)

Production of bulk carbon nanotube materials is of particular interest.Such carbon nanotube materials can have particularly beneficialproperties, such as relatively low density and high strength.

WO2008/132467 describes densifying carbon nanotubes to improve theefficiency of carbon nanotube packing, in order to provide a fibre orfilm. For example, a density enhancement agent such as divinyl benzenemay be applied to the carbon nanotubes, in order to improve the packingof the carbon nanotubes, which provides a higher strength material. Thefibres and films described in WO2008/132467 may be at least one metrelong.

Similarly, Koziol et al′ have described the production of carbonnanotube fibres with high specific strength and high specific stiffness.This document describes the production of carbon nanotubes by thermalchemical vapour deposition (CVD). In the methods described, theresulting “aerogel” of carbon nanotubes is drawn into a fibre, which isrun through an acetone vapour stream to enhance the densification. Awinding rate of up to 20 m min⁻¹ is employed to draw the fibre.

Motta et al² have described the effect of sulphur as a promoter ofcarbon nanotube formation. They describe using thiophene as a sulphurprecursor in iron catalysed thermal CVD, to produce nanotubes with adiameter of between 4 nm and 10 nm, which were typically double-walled.The iron catalyst particles were about 5 nm to 10 nm. The resultingaerogel was drawn into a fibre, with a winding rate of 20 m min⁻¹. Mottaet al report a high carbon nanotube growth rate of up to 0.1 to 1 mmsec⁻¹.

The carbon materials produced by the methods described in thesedocuments include a mixture of carbon nanotubes with a wide distributionof diameters and with a wide distribution of chiralities (includingarmchair, zigzag and intermediate chiralities). Increasing the degree ofcontrol of carbon nanotube formation would provide a greater control ofthe properties of the resulting carbon materials produced, but althoughmany researchers have made efforts to provide such control, the presentinventors are not aware of any disclosure of significant recent progressin this area.

SUMMARY OF THE INVENTION

The present inventors have devised the present invention in order toaddress one or more of the above problems.

The present inventors have realised that by ensuring close control ofthe size of catalyst particles in gas phase formation of carbonnanotubes, for example in chemical vapour deposition (CVD), it ispossible to control the diameter of the carbon nanotubes produced. Theresulting carbon nanotubes typically exhibit armchair chirality andtypically have metallic properties. The carbon nanotubes produced inthis method readily form bulk materials, for example by thedensification methods described by Koziol et al′.

Thus, the present inventors have for the first time demonstrated that itis possible to produce an electrically conductive carbon material inbulk form, which includes a narrow size range of small diameter carbonnanotubes.

Accordingly, in a first preferred aspect, the present invention providesa carbon material comprising carbon nanotubes, wherein at least 70% bynumber of the carbon nanotubes have a diameter in the range from 1 nm to2.5 nm.

The term “material” here is intended to mean a substance that has theform of a solid and has independent existence, in the sense that it hasno requirement to be supported by a substrate. Thus, the material can beself-supporting (or, more generally, capable of being self-supporting).However, the materials of the invention may cooperate with othermaterials (such as substrates) in order to provide the materials withadditional functionality.

The carbon material is preferably provided in the form of at least onefibre. The fibre typically comprises a very large number of carbonnanotubes.

The carbon material preferably has a conductivity of at least 0.7×10⁶Sm⁻¹ in at least one direction (at room temperature). Preferably, thecarbon material comprises at least 75% by weight of carbon nanotubes.The carbon material may be, for example, a fibre or a film. It may haveat least one dimension greater than 0.5 m.

In a second preferred aspect, the present invention provides a method ofproducing carbon nanotubes, the method comprising:

-   -   providing a plurality of floating catalyst particles, wherein at        least 70% by number of the catalyst particles have a diameter        less than or equal to 4.5 nm; and    -   contacting the floating catalyst particles with a gas phase        carbon source to produce carbon nanotubes.

The significance of contacting floating catalyst particles with the gasphase carbon source is that, at least during the formation of the carbonnanotubes, the catalyst particles are not supported on a substrate butinstead are held (e.g. suspended) within a gas.

Preferably, the step of providing a plurality of catalyst particlescomprises initiating growth of catalyst particles and subsequentlyarresting the growth of the catalyst particles using an arresting agent.The steps of initiating growth of catalyst particles and subsequentlyarresting their growth is preferably carried out in the gas phase.

In another preferred aspect, the present invention provides a method ofproducing carbon nanotubes, the method comprising:

-   -   providing a plurality of floating catalyst particles; and    -   contacting the floating catalyst particles with a gas phase        carbon source to produce carbon nanotubes

wherein the floating catalyst particles are provided by:

-   -   initiating growth of the catalyst particles by thermal        degradation of a catalyst source substance, the thermal        degradation of the catalyst source substance beginning at a        first onset temperature, and subsequently    -   arresting the growth of the catalyst particles using an        arresting agent, the arresting agent being provided to the        catalyst particles by thermal degradation of an arresting agent        source substance, the thermal degradation of the arresting agent        source substance beginning at a second onset temperature,        wherein the second onset temperature is in the range of        temperatures from 10° C. more than the first onset temperature        to 350° C. more than the first onset temperature.

Further preferred temperature ranges are set out below.

WO2010/014650 reports the preparation of metallic single-wall carbonnanotubes. This document describes dispersing Fe-containing catalystparticles on a substrate, then treating the catalyst particles to obtainthe desired catalyst particle size, for example an average particlediameter ranging from 0.2 nm to 5 nm, or from about 0.9 nm to 1.4 nm.The catalyst particles, which are immobilised on the substrate, are thencontacted with a gaseous carbon source, to produce carbon nanotubeswhich are correspondingly immobilised on the substrate. This documentreports the production of predominantly metallic single wall nanotubes,under certain reaction conditions. However, the formation of carbonnanotubes on a substrate does not provide a route for the production ofa carbon nanotube material, such as an electrically conductive material.A similar method is described by Harutyunyan et al³.

Gas phase production of carbon nanotubes typically results in a lowdensity mass of carbon nanotubes. A typical density of this mass ofcarbon nanotubes is less than 10⁻¹ g cm⁻³, or less than 10⁻² g cm⁻³.

In the literature, such a mass of carbon nanotubes is sometimes referredto as an “aerogel”, although the use of this term is not systematicallyapplied. Such a mass of carbon nanotubes can be densified to provide acarbon material, such as a fibre or film. However, alternatively, themass of carbon nanotubes may be crushed, chopped, cut or otherwiseprocessed to form a powder. It will be understood that the powder maynot exhibit significant electrical conductivity.

Accordingly, in a third preferred aspect, the present invention providesa carbon powder comprising carbon nanotubes, wherein at least 70% bynumber of the carbon nanotubes have a diameter in the range from 1 nm to2.5 nm.

Preferably, the carbon powder is provided in an amount of at least 10 g.

In a further preferred aspect, the present invention provides a carbonmaterial or a carbon nanotube powder obtained or obtainable by themethod of the second preferred aspect. It will be understood that thecarbon materials and the carbon powders described herein may be obtainedor obtainable by the methods of making carbon nanotubes describedherein.

In a further preferred aspect, the present invention provides a currentcarrying component comprising a carbon material according to the firstpreferred aspect.

In a further preferred aspect, the present invention provides a currentcarrying component consisting of: carbon nanotubes; optionally,remaining catalyst particles; and incidental impurities, wherein atleast 70% by number of the carbon nanotubes have a diameter in the rangefrom 1 nm to 2.5 nm.

The current carrying component preferably has a length of at least 0.5m, more preferably (for some embodiments) at least 1 m, at least 10 m orat least 100 m. The current carrying component may, for example, beprovided in the form of an electrical cable, an electrical interconnector an electrical wire. The diameter of the current carrying component isnot particularly limited in the present invention, but will typically bedetermined by the application to which the component will be put, takinginto account the required current carrying capacity for thatapplication. The current carrying component is preferably to be used ator near ambient temperature.

The current carrying component may be used in a range of electricalapplications. The current carrying component may be used in a powertransmission cable. The current carrying component may be used in alightning protection system. Alternatively, the current carryingcomponent may be used in general electrical wiring applications, e.g. toreplace conventional copper wiring. In a preferred embodiment, thecurrent carrying component may be used as the current-carrying windingsof an electromagnet, for example in a solenoid or more preferably in anelectric motor. The combination of properties of the preferred currentcarrying components (high current density, high strength, low density)are particularly well suited to the manufacture of small size and/or lowweight electric motors.

In a further preferred aspect, the present invention comprises a wovenor non-woven fabric, comprising a carbon material according to the firstpreferred aspect.

In a further preferred aspect, the present invention provides a woven ornon-woven fabric consisting of: carbon nanotubes; optionally, remainingcatalyst particles; and incidental impurities, wherein at least 70% bynumber of the carbon nanotubes have a diameter in the range from 1 nm to2.5 nm.

The woven or non-woven fabric may comprise a plurality of fibres, eachfibre being formed of a large number of carbon nanotubes.

The woven or non-woven fabric may be used in clothing. For example, theclothing may include sensors, for monitoring the condition of thewearer. The sensors may be arranged to transmit information from thesensors to a remote receiver, providing for remote monitoring of thecondition of the wearer, for example remote monitoring of the health ofthe wearer. For example, the clothing may provide for remote monitoringof the temperature of the wearer. The clothing may be worn, for example,by a patient or by a soldier. It will be understood that theelectrically conductive fibres of the woven or non-woven fabric may formpart of the sensor and information transmission system.

Further preferred or optional features of the above aspects will now beset out. Any aspect of the invention may be combined with any otheraspect, unless the context demands otherwise. Any of the preferred oroptional features of any aspect may be combined, either singly or incombination, with any aspect of the invention, unless the contextdemands otherwise. Where a series of end points for a particular rangeis given, it is to be understood that any one of those end points can beapplied independently to the invention.

The carbon material of the present invention is electrically conductive.Preferably, it has a conductivity of at least 0.7×10⁶ S m⁻¹ in at leastone direction (at room temperature). More preferably, it has aconductivity of at least 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6,1.7, 1.8, 1.9, or 2.0×10⁶S m⁻¹ in at least one direction (at roomtemperature). It is preferred that the carbon nanotubes dominate theelectrical properties of the material, thus providing the material withits electrical conductivity.

Preferably, at least at room temperature, the carbon material has apositive coefficient of resistivity with increasing temperature.

The carbon material may have a current density of at least 15 A mm⁻²,more preferably at least 20, at least 25, at least 30, at least 35, atleast 40, at least 50, at least 60 or at least 70 A mm⁻². As usedherein, the term “current density” refers to the amount of currentdensity which can be carried by the carbon material without requiringforce cooling to avoid runaway heating.

The carbon material may preferably be a fibre or a film. Where thecarbon material is a fibre, the carbon nanotubes may have theirprincipal axes substantially aligned with the length direction of thefibre. Similarly, where the carbon material is a film, the principalaxes of the carbon nanotubes may be substantially aligned with eachother and may lie substantially in the plane of the film. The carbonmaterial may comprise bundles of carbon nanotubes, in which bundles theprincipal axes of the carbon nanotubes may be substantially aligned witheach other.

The carbon material may be a yarn, comprising bundles of fibres (whichfibres may comprise bundles of carbon nanotubes). It will be understoodthat the yarn may consist of bundles of fibres, optionally remainingcatalyst particles, and incidental impurities.

The carbon material preferably has at least one dimension greater than0.5 m. The carbon material may have at least one dimension greater than1 m, 2 m, 5 m, 10 m, 15 m or 20 m.

Where the carbon material is a fibre or a yarn, said at least onedimension may be the length of the fibre. Where the carbon material is afibre, typically the fibre has a diameter in the range from 1 μm to 10cm. More preferably, the fibre has a diameter in the range from 1 μm to1 mm, or from 1 μm to 100 μm, or from 1 μm to 50 μm. A typical fibrediameter is 10 μm.

Where the carbon material is a film, said at least one direction may bethe length of the film. The film may have a thickness of at least 10 nm,for example at least 20 nm, at least 30 nm or at least 40 nm. The filmmay have a thickness of 1 mm or less, more preferably 500 μm or less,250 μm or less, 100 μm or less, 1 μm or less, or 100 nm or less. Atypical thickness is 50 nm. It will be understood that two or more filmsmay be placed on top of each other e.g. to provide a plurality ofoverlying layers, which may together have a thickness greater than thoseset out above.

A particular advantage of the carbon material of the present inventionis that it may provide a relatively high electrical conductivity whilehaving relatively low density, compared for example with metals andalloys typically employed as electrical current carrying components.Typically, the carbon material has a density of 0.1 g cm⁻³ or more. Itmay have a density of at least 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 or 0.8 gcm⁻³ or more. Preferably, the carbon material has a density of 2.0 gcm⁻³ or less, such as a density of 1.5, 1.4, 1.3, 1.2 or 1.1 g cm⁻³ orless. In contrast, aluminium typically has a density of 2.7 g cm⁻³, andcopper typically has a density of 8.9 g cm⁻³.

Where the carbon material is a fibre, its linear density may instead beconsidered. For example, it may have a linear density which is 1 g km⁻¹or less, for example 0.5, 0.4, 0.3, 0.2, 0.1 or 0.05 g km⁻¹ or less.Such a low linear density may be suitable for some specificapplications. However, it is to be understood that other specificapplications (e.g. electrical power cabling applications) will require amuch higher linear density.

Another advantage of the carbon materials of the present invention isthat they may provide a relatively high electrical conductivity combinedwith a relatively high strength. The carbon material preferably has aspecific strength of at least 0.1 GPa SG⁻¹ in at least one direction,such as at least 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 or 0.8 GPa SG⁻¹. Incontrast, aluminium typically has a specific strength of 0.026 GPa SG⁻¹,and copper typically has a specific strength of 0.025 GPa SG⁻¹.

(As used herein, specific strength is the ultimate tensile strength(UTS, measured in GPa) of the material concerned, divided by itsspecific gravity (SG). Specific gravity is a dimensionless value,obtained by dividing the density of the substance in question by thedensity of a reference substance, in this case water. The calculation ofspecific strength is explained in detail Reference 1, which isincorporated herein by reference in its entirety.)

Similarly, the specific stiffness of the carbon materials of the presentinvention is relatively high. The carbon material preferably has aspecific stiffness of 30 GPa SG⁻¹ or more, more preferably 40 or 50 GPaSG⁻¹ or more. In contrast, aluminium typically has a specific stiffnessof 26 GPa SG⁻¹, and copper typically has a specific stiffness of 13 GPaSG⁻¹. (Here, stiffness is the elastic modulus of the material, andspecific stiffness is determined by dividing this value by the specificgravity of the material. The calculation of specific stiffness isexplained in detail Reference 1, which is incorporated herein byreference in its entirety.)

Preferably, the carbon material or the carbon nanotube powder comprisesat least 75% by weight of carbon nanotubes. It may comprise at least80%, 85%, 90%, 95%, 96%, 97%, 98% or 99% by weight of carbon nanotubes.

It will be understood that the carbon material of the present invention,and the carbon nanotube powder of the present invention, may compriseother components. For example, residual catalyst particles employed inthe synthesis of the carbon nanotubes may remain in the carbon material.Accordingly, the carbon material of the present invention, and thecarbon nanotube powder of the present invention, may comprise aplurality of catalyst particles dispersed in the material. Preferably,the material or powder comprises 20% by weight or less of catalystparticles, for example 15%, 10%, 5%, 4%, 3%, 2% or 1% by weight or lessof catalyst particles.

The catalyst particles may have any of the features described below inrelation to the catalyst particles employed in the methods of thepresent invention. For example, the catalyst particles may comprisetransition metal atoms, such as iron, cobalt and/or nickel atoms. Thecatalyst particles may comprise sulphur atoms. In a particularlypreferred embodiment, the catalyst particles may comprise an inner coreof transition metal atoms surrounded by a shell or cage of sulphuratoms.

At least 70% by number of the catalyst particles may preferably have adiameter less than or equal to 4.5 nm, or less than or equal to 3.5 nm.For example, at least 70% of the catalyst particles may have a diameterin the range from 0.5 nm to 4.5 nm, more preferably from 0.5 nm to 3.5nm, or from 1.5 nm to 3.5 nm. More preferably, at least 75%, 80%, 85%,90%, 95%, 96%, 97%, 98% or 99% by number of the catalyst particles havea diameter less than or equal to 4.5 nm, or less than or equal to 3.5nm. It will be understood that at least 75%, 80%, 85%, 90%, 95%, 96%,97%, 98% or 99% by number of the catalyst particles may have a diameterin the range from 0.5 nm to 4.5 nm, more preferably from 0.5 nm to 3.5nm, or from 1.5 nm to 3.5 nm. The number and size of catalyst particlesmay be determined by transmission electron microscopy (TEM), asdescribed in more detail below with reference to the methods of thepresent invention.

The carbon material may, for example, consist of a plurality of carbonnanotubes, optionally a plurality of catalyst particles dispersed in thematerial, and incidental impurities. Similarly, the carbon nanotubepowder may consist of a plurality of carbon nanotubes, optionally aplurality of catalyst particles in the powder, and incidentalimpurities.

As described above, the methods of the present invention can provide ahigh degree of control of carbon nanotube diameter, chirality andmetallic properties. Typically, the methods result in the production ofa population of carbon nanotubes having a high proportion of metalliccarbon nanotubes. Typically, the methods result in the production of apopulation of carbon nanotubes having a high proportion of armchaircarbon nanotubes.

Accordingly, the present inventors have made available for the firsttime carbon materials comprising a high proportion of metallic carbonnanotubes. Accordingly, it will be understood that the carbon materialsof the present invention, and the carbon nanotube powders, typicallycomprise a high proportion of metallic carbon nanotubes. For example,substantially all of the carbon nanotubes may be metallic.

Similarly, the present inventors have made available for the first timecarbon materials comprising a high proportion of armchair carbonnanotubes. Accordingly, it will be understood that the carbon materialsof the present invention, and the carbon nanotube powders, typicallycomprise a high proportion of armchair carbon nanotubes. For example,substantially all of the carbon nanotubes may have armchair chirality.

The person skilled in the art will be familiar with methods for probingthe metallic or semiconducting properties of carbon nanotubes. Onesuitable method for probing the metallic or semiconducting properties ofcarbon nanotubes, for example in a carbon material or carbon nanotubepowder, employs Raman spectroscopy.

One vibrational mode of carbon nanotubes is the radial breathing mode.This radial breathing mode can be probed using Raman spectroscopy. For agiven wavelength of incident light, only radial breathing modes ofcarbon nanotubes with certain diameters will be resonant, and so onlycertain diameters of carbon nanotubes will give rise to radial breathingmode (RBM) peaks in the Raman spectrum. The wavenumber of a given RBMpeak can be used to determine the diameter of the carbon nanotube whichgave rise to that peak, using the equation d=239/ω_(RBM), wherein d isthe nanotube diameter in nm, and ω_(RBM) is the wavenumber of the radialbreathing mode peak in cm⁻¹.

Once the diameter of the carbon nanotube(s) giving rise to the RBM peakhas been determined, it is possible to determine whether those carbonnanotube(s) are metallic or semiconducting. This is done using a plotcalled a Kataura plot, such as the plot shown in FIG. 1. The diameter ofthe carbon nanotube is read from the x-axis, and the excitation energyused to generate the RBM peak in question is read from the y-axis. For agiven diameter and excitation energy, the plot indicates whether thecarbon nanotube(s) giving rise to the RBM peak are metallic orsemiconducting.

For example, a typical method of probing the metallic or semiconductingproperties of carbon nanotubes in a carbon material or powder comprisesthe steps of:

-   -   (i) taking a first Raman spectrum using an incident wavelength        of 633 nm;    -   (ii) identifying each peak falling in the range from 120 cm⁻¹ to        350 cm⁻¹ (RBM peaks);    -   (iii) determining the position (ω_(RBM)) of each RBM peak using        Lorentzian fit;    -   (iv) determining the nanotube diameter associated with each RBM        peak, using the equation

d=239/ω_(RBM)

-   -   wherein d is the nanotube diameter in nm, and ω_(RBM) is the        frequency of the radial breathing mode peak in cm⁻¹;    -   (v) comparing this diameter with the Katura plot shown in FIG.        1, using an excitation energy of 1.96+/−0.1 eV (which        corresponds to the 633 nm incident light) to determine whether        each RBM peak corresponds to metallic or semiconducting carbon        nanotubes;    -   (vi) taking a second Raman spectrum using an incident wavelength        of 514 nm, corresponding to an excitation energy of 2.41+/−0.1        eV, and repeating steps (ii) to (v) for this second Raman        spectrum.

In FIG. 1, the filled circles indicate semiconducting nanotubes, and theopen circles indicate metallic nanotubes.

When the above method is carried out on the carbon material or carbonnanotube powder of the present invention, preferably at least one of thefirst and second Raman spectra includes at least one RBM peak whichcorresponds to metallic carbon nanotubes. Preferably, neither of thefirst and second Raman spectra includes any RBM peak which correspondsto semiconducting carbon nanotubes.

In the above method for probing the metallic or semiconductingproperties of carbon nanotubes, the sample probed may be a bulk carbonmaterial. In that case, preferably the method is carried out a pluralityof times, on different regions of the sample. For example, preferably atleast 10 regions are probed. Preferably, at least 70%, 80% or 90% of theprobed regions meet one or more of the conditions recited above. In thecase of a fibre, the incident light may be aligned with the fibre axis.The regions probed may be equally spaced, e.g. at a spacing distance of1 cm along the fibre axis.

Alternatively, the carbon nanotubes of the carbon material may bedispersed before the method for probing the metallic or semiconductingproperties of carbon nanotubes is carried out. In this case, the methodmay be carried out on a single sample of dispersed carbon nanotubes.Alternatively, the method may be repeated for one or more samples ofdispersed carbon nanotubes, for example 10 samples. Preferably, at least70%, 80% or 90% of the probed samples meet one or more of the conditionsrecited above.

In the case of a carbon nanotube powder, the method of probing themetallic or semiconducting properties of carbon nanotubes can be carriedout on one or more, e.g. 10 samples. Preferably, at least 70%, 80% or90% of the probed samples meet one or more of the conditions recitedabove.

A suitable laser source for the 633 nm incident light is He/Ne. Asuitable laser source for the 514 nm incident light is Ar ion. The rangeof wavenumbers scanned for each spectrum may be at least 100 cm⁻¹ to 400cm⁻¹, e.g. 50 cm⁻¹ to 3300 cm⁻¹. A suitable spectroscope is the RenishawRamanscope 1000 system, available from Renishaw (www.renishaw.com). Asuitable laser spot size is 1 μm². A suitable acquisition time is 10 s.

The Raman spectroscopy methods described above are particularly suitedto probing single-walled carbon nanotubes. It is preferable that thecarbon material, or the carbon nanotube powder, comprises single-walledcarbon nanotubes. Preferably, at least 50% by number of the carbonnanotubes are single-walled carbon nanotubes. More preferably, at least60%, 70%, 80%, 90%, 95%, 96%, 97%, 98% or 99% by number of the carbonnanotubes are single-walled carbon nanotubes.

Methods of probing the properties of carbon nanotubes using Ramanspectroscopy are described in Reference 4 and Reference 5, which areeach incorporated herein by reference in their entirety.

As explained above, the carbon materials of the present invention, andthe carbon nanotube powders, typically comprise a high proportion ofarmchair carbon nanotubes. For example, substantially all of the carbonnanotubes may have armchair chirality.

The chirality of a single carbon nanotube, of a bundle of carbonnanotubes or, more generally, of a population of carbon nanotubes (e.g.in a carbon material or carbon nanotube powder) can be probed usingelectron diffraction. As the skilled person understands, TEM analysisallows the production of electron diffraction patterns by suitableoperation of the microscope. The electron beam is directed through thecarbon nanotube (or bundle of carbon nanotubes) in a directionperpendicular to the principal axis of the carbon nanotube. Theresultant electron diffraction pattern indicates the chirality of thecarbon nanotube.

For both zigzag and armchair single-walled nanotubes, a hexagonalpattern of six diffraction spots is generated. However, the orientationof these spots with respect to the principal axis of the nanotube isdifferent for zigzag and for armchair nanotubes. For armchair nanotubes,three of the six spots are positioned to one side of the principal axisof the nanotube, and three are positioned to the other side of theprincipal axis. In contrast, for zigzag carbon nanotubes, two of the sixspots are positioned to one side of the principal axis, two arepositioned to the other side of the principal axis, and two spots arealigned with the principal axis.

This is illustrated in FIG. 2. FIG. 2A shows a schematic representationof the diffraction pattern generated by an armchair carbon nanotube, andFIG. 2B shows a schematic representation of the diffraction patterngenerated by a zigzag carbon nanotube. The “x” points indicate theposition of the diffraction spots. The solid vertical lines illustratethe principal axis direction of the carbon nanotube.

Where a population (e.g. a bundle) of carbon nanotubes having the samechirality is probed, similar results are observed. Where a mixture ofdifferent chiralities is present, the diffraction spots generated bycarbon nanotubes of different chiralities together form a circularpattern of diffraction spots. In some cases, these spots may merge toform a continuous circular diffraction pattern. Similarly, inmulti-walled carbon nanotubes, typically a circular diffraction patternis generated, indicating a mixture of chiralities.

In all cases, the spots tend to be slightly elongated streaks, elongatedin a direction substantially perpendicular to the principal axis of thenanotube, in the plane of the diffraction pattern.

Preferably, at least 50% by number of the carbon nanotubes aresingle-walled armchair carbon nanotubes. More preferably, at least 60%,70%, 80%, 90%, 95%, 96%, 97%, 98% or 99% by number of the carbonnanotubes are single-walled armchair carbon nanotubes.

The percentage by number of single-walled carbon nanotubes havingarmchair chirality can be determined using electron diffraction, asdescribed above. 50% by number of the carbon nanotubes are considered tobe armchair single-walled carbon nanotubes wherein when at least 10separate regions of the material are probed by electron diffraction(e.g. 10 different carbon nanotubes (not in the same bundle), or 10different bundles of carbon nanotubes) using a beam perpendicular to theprincipal axis of the carbon nanotube, at least 50% of the probednanotubes or bundles give rise to an armchair diffraction pattern. Anarmchair diffraction pattern is considered to be a hexagonal pattern ofsix diffraction spots, where three of the six spots are positioned toone side of the principal axis of the carbon nanotube, and three arepositioned, substantially mirror symmetrically, to the other side of theprincipal axis of the carbon nanotube.

More preferably, an armchair diffraction pattern may be considered to bea hexagonal pattern of six diffraction spots, where the spots arelocated at positions 30°, 90°, 150°, 210°, 270° and 330°+/−5° (or +/−4°,3°, 2° or 1°) with respect to the principal axis of the carbon nanotube.Here, the positional angle is measured between the principal axis of thecarbon nanotube and a line extending from the centre of the beam(central spot) to the centre of the diffraction spot.

Where a bundle of nanotubes is probed, the nanotubes should preferablyhave their principal axes aligned. Conveniently, where the carbonmaterial is a fibre, the fibre axis may be taken to be the principalaxis of the carbon nanotubes, although this can of course be easilyconfirmed during the TEM analysis.

(It will be understood that the principal axis of a carbon nanotubeextends along the elongation direction of the carbon nanotube.)

Suitable armchair chiralities for the carbon nanotubes of the materialand powder of the present invention include the following. Thechiralities are given in terms of n and m (n, m) and are listedalongside their diameter:

(3, 3) 0.41 ± 0.02 nm (4, 4) 0.54 ± 0.02 nm (5, 5) 0.68 ± 0.02 nm (6, 6)0.81 ± 0.02 nm (7, 7) 0.95 ± 0.02 nm (8, 8) 1.10 ± 0.02 nm (9, 9) 1.22 ±0.02 nm (10, 10) 1.36 ± 0.02 nm (11, 11) 1.49 ± 0.02 nm (12, 12) 1.63 ±0.02 nm (13, 13) 1.76 ± 0.02 nm (14, 14) 1.90 ± 0.02 nm (15, 15) 2.03 ±0.02 nm (16, 16) 2.17 ± 0.02 nm (17, 17) 2.31 ± 0.02 nm (18, 18) 2.44 ±0.02 nm (19, 19) 2.58 ± 0.02 nm (20, 20) 2.71 ± 0.02 nm (21, 21) 2.85 ±0.02 nm (22, 22) 2.98 ± 0.02 nm

The (n, m) notation employed here will be familiar to the person skilledin the art.

In the carbon materials and carbon nanotube powder of the presentinvention, preferably at least 70% by number of the carbon nanotubeshave a diameter in the range from 1 nm to 2.5 nm, more preferably from 1nm to 2 nm. More preferably, at least 75%, 80%, 85%, 90%, 95%, 96%, 97%,98% or 99% by number of the carbon nanotubes have a diameter in therange from 1 nm to 2.5 nm, more preferably from 1 nm to 2 nm. Thediameter of carbon nanotubes in a material can be determined by TEM. Thesize distribution of the carbon nanotubes can be determined by counting.For example, TEM may be carried out on 2, 5, 10 or more samples of thecarbon material or carbon nanotube powder to determine the sizedistribution.

As set out above, in at least one aspect, the present invention providesa method of producing carbon nanotubes, the method comprising:

-   -   providing a plurality of floating catalyst particles, wherein at        least 70% by number of the catalyst particles have a diameter        less than or equal to 4.5 nm; and    -   contacting the floating catalyst particles with a gas phase        carbon source to produce carbon nanotubes.

The method typically yields a mass of carbon nanotubes, having a densityof 10⁻² g cm⁻³ or less, for example 10⁻³ g cm⁻³ or less. Typically, themass of carbon nanotubes has a density of 10⁻⁶ g cm⁻³ or more. In theliterature, this mass of carbon nanotubes is sometimes referred to as anaerogel.

At least 70% by number of the catalyst particles have a diameter whichis less than or equal to 4.5 nm, or less than or equal to 3.5 nm. Forexample, at least 70% by number of the catalyst particles have adiameter in the range from 0.5 nm to 4.5 nm, more preferably from 0.5 nmto 3.5 nm or from 1.5 nm to 3.5 nm. More preferably, at least 75%, 80%,85%, 90%, 95%, 96%, 97%, 98% or 99% by number of the catalyst particleshave a diameter less than or equal to 4.5 nm or less than or equal to3.5 nm. It will be understood that at least 75%, 80%, 85%, 90%, 95%,96%, 97%, 98% or 99% by number of the catalyst particles may have adiameter in the range from 0.5 nm to 4.5 nm, more preferably from 0.5 nmto 3.5 nm or from 1.5 nm to 3.5 nm.

The diameter of the catalyst particles can be determined by TEM. Thediameter of a catalyst particle is taken to be the largest lineardimension of that particle visible on the TEM image. Typically, theproduct of the process for forming the carbon nanotubes includes notonly the carbon nanotubes but also residual catalyst particles,typically randomly dispersed amongst the population of carbon nanotubes.Accordingly, the TEM can be carried out on a sample of carbon nanotubesproduced by the process, and thus the size distribution of catalystparticles determined. Alternatively, catalyst particles can be isolatedduring the process, after their formation but before carbon nanotubeproduction. TEM can be carried out on the isolated catalyst particles todetermine their diameter.

The present inventors have realised that the size of the catalystparticles can be conveniently controlled by initiating growth ofcatalyst particles and subsequently arresting the growth of the catalystparticles using an arresting agent. This may be carried out in the gasphase.

The growth of catalyst particles may be initiated by degradation of acatalyst source substance (e.g. a catalyst source compound or element),and/or the arresting agent may be supplied by degradation of anarresting agent source substance (e.g. an arresting agent sourcecompound or element). Typically, this may be performed by:

-   -   subjecting a mixture of catalyst source substance and arresting        agent source substance to catalyst source substance degradation        conditions;    -   and subsequently subjecting said mixture to arresting agent        source substance degradation conditions.

It will be understood that the term “degradation” as used hereinincludes chemical breakdown of a compound to release e.g. its componentatoms or a simpler compound. It will be understood that the term“degradation” as used herein also includes physical change in asubstance which results in the release of a catalyst component such as atransition metal atom (to allow catalyst particle growth), or release ofarresting agent (to arrest catalyst particle growth). For example, thephysical change could be vaporisation or sublimation. The term“degradation conditions” should be interpreted accordingly.

Typically, degradation of the catalyst source substance (e.g. compound)may be by thermal degradation. Typically, the degradation of arrestingagent source substance (e.g. compound) may be by thermal degradation.This is desirable in embodiments where the carbon nanotubes are producedby thermal chemical vapour deposition (thermal CVD). However, it isnoted here that other manufacturing conditions may be used, e.g. plasmaCVD.

Thermal degradation of the catalyst source substance may begin at afirst onset temperature, and thermal degradation of the arresting agentsource substance may begin at a second onset temperature. Preferably,the second onset temperature is greater than the first onsettemperature. Preferably, the second onset temperature is not more than350° C. greater than the first onset temperature, for example not morethan 300° C., 250° C., 200° C., 150° C., 100° C., 90° C., 80° C., 70°C., 60° C. or 50° C. greater than the first onset temperature.Preferably, the second onset temperature is at least 10° C. greater thanthe first onset temperature, for example at least 20° C., 30° C., 40°C., 50° C., 60° C., 70° C., 80° C., 90° C. or 100° C. greater than thefirst onset temperature.

The first onset temperature may be at least 200° C., or at least 300° C.The first onset temperature may be 700° C. or less, more preferably 600°C. or less, 500° C. or less, or 400° C. or less. The second onsettemperature may be at least 300° C., at least 400° C., at least 450° C.,or at least 500° C. The second onset temperature may be 800° C. or less,or 700° C. or less, or 650° C. or less, or 600° C. or less, or 550° C.or less, or 500° C. or less, or 450° C. or less.

The carbon nanotubes may be produced at a carbon nanotube formationtemperature. This is typically higher than the second onset temperature.Preferably, the carbon nanotube formation temperature is at least 900°C., for example at least 950° C., 1000° C., 1050° C., 1100° C. or 1150°C.

Preferably, the steps of:

-   -   initiating growth of catalyst particles;    -   subsequently arresting the growth of the catalyst particles        using an arresting agent; and    -   contacting the catalyst particles with a carbon source to        produce carbon nanotubes

are all carried out in the same reaction chamber.

Each (e.g. all) of these steps may be carried out in the gas phase.

The arresting agent source substance, the catalyst source substance andthe carbon source may pass through the reaction chamber in a flowdirection. For example, the arresting agent source substance, thecatalyst source substance and the carbon source may pass through thechamber in the gas phase, e.g. in said flow direction. The arrestingagent source substance, the catalyst source substance and the carbonsource may be carried through the chamber as part of a gas stream. Thegas stream may include an inert gas, such as a noble gas, for examplehelium or argon. The gas stream may include a reductive gas, for examplehydrogen.

The conditions inside the reactor may vary along the flow direction. Forexample, the temperature in the reaction chamber may vary along the flowdirection. The temperature may increase from the first onset temperatureto the second onset temperature along the flow direction. Thetemperature may then change (e.g. increase) to the carbon nanotubeformation temperature. For example, the reaction chamber may be afurnace.

Preferably, the catalyst particles comprise transition metal atoms. Forexample, the catalyst particles may comprise iron, cobalt and/or nickelatoms, preferably iron atoms. Accordingly, it may be preferred that thecatalyst source substance (e.g. compound) comprises at least onetransition metal atom, for example at least one iron atom, at least onenickel atom and/or at least one cobalt atom. Preferably, the catalystsource substance (e.g. compound) comprises at least one iron atom. Theseatoms may be released on degradation of the catalyst source substance(e.g. compound).

(As used herein, the word “atom” is understood to include ions of therelevant atoms. For example, the catalyst particles and/or the catalystsource substance (e.g. compound) may include one or more transitionmetal ions.)

For example, the catalyst source substance may be a transition metalcomplex, for example a transition metal complex including one, orpreferably two, cyclopentadienyl ligands. Alternatively or additionally,the transition metal complex may include other ligands, such as one ormore carbonyl ligands. The transition metal complex may include onlyhydrocarbon ligands.

For example, the catalyst particle source substance may be ferrocene.Other suitable catalyst particle source substances include othermetalocenes, such as nickelocene and cobaltocene. Metal carbonylcompounds are also suitable, for example cobalt carbonyl (e.g. dicobaltoctacarbonyl), nickel carbonyl (e.g. nickel tetracarbonyl) and ironcarbonyl (e.g. iron pentacarbonyl). It will be understood thattransition metal complexes having a mixture of cyclopentadienyl ligandsand carbonyl ligands may also be suitable.

It will be understood that the catalyst source substance is preferably asubstance which begins thermal degradation at the first onsettemperature, for example as set out above. Preferably, the catalystsource substance begins thermal degradation at the first onsettemperature under the conditions employed in the method of theinvention, for example under reductive conditions.

Preferably, the arresting agent is sulphur. Preferably, the arrestingagent source substance (e.g. compound) comprises at least one sulphuratom. The sulphur atom may be released on degradation of the arrestingagent source substance. The arresting agent source substance may be acompound comprising at least one sulphur atom and at least one carbonatom covalently bonded to the sulphur atom. In this case, at least onecarbon-sulphur covalent bond may be broken on degradation of thearresting agent source substance, in order to release the sulphur atom.

A typical arresting agent source substance is carbon disulphide (CDS).

Alternatively, the arresting agent source substance may be a compoundcomprising at least one sulphur atom and at least one hydrogen atomcovalently bonded to the sulphur atom. In this case, at least onehydrogen-sulphur covalent bond may be broken on degradation of thearresting agent source substance, in order to release the sulphur atom.Accordingly, it will be understood that a further suitable arrestingagent is hydrogen sulphide (H₂S).

A further suitable arresting agent source substance is elementalsulphur. For example, solid sulphur may be supplied to the reactionchamber. Sulphur atoms, ions, radicals or molecules may be releasedwithin the reaction chamber, for example by vaporisation or sublimationof the elemental sulphur (e.g. solid sulphur). It will be understoodthat this release of sulphur atoms, ions, radicals or molecules, e.g. bysublimation or vaporisation, can be considered to be arresting agentsource substance degradation (e.g. thermal degradation). Alternatively,gaseous sulphur may be supplied to the reaction chamber.

It will be understood that the arresting agent source substance (e.g.compound) is preferably a substance which begins thermal degradation atthe second onset temperature, for example as set out above. Preferably,the arresting agent source substance (e.g. compound) begins thermaldegradation at the second onset temperature under the conditionsemployed in the method of the invention, for example under reductiveconditions.

It will be understood that the catalyst particles may comprisetransition metal atoms, such as iron, cobalt and/or nickel atoms. Thecatalyst particles may comprise sulphur atoms. The catalyst particlesmay consist of transition metal atoms such as iron atoms, sulphur atoms,and incidental impurities. In a particularly preferred embodiment, thecatalyst particles may comprise an inner core of transition metal atomssurrounded by a shell or cage of sulphur atoms.

The degradation of catalyst source substance and/or the degradation ofarresting agent source substance may be carried out under reductiveconditions, for example in the presence of hydrogen.

The amount of catalyst source substance and arresting agent sourcesubstance supplied may provide a molar ratio of transition metal atomsto sulphur atoms of 50:1 or less, more preferably 40:1, 30:1, 20:1, 15:1or 10:1 or less. Preferably, the molar ratio of transition metal atomsto sulphur atoms is at least 2:1, more preferably at least 3:1, at least4:1 or at least 5:1. A typical molar ratio of transition metal atoms tosulphur atoms is 6:1.

A typical molar ratio of carbon atoms to transition metal atoms is 8:1.Preferably, the ratio of carbon atoms to transition metal atoms is atleast 2:1, at least 3:1, at least 4:1, or at least 5:1. Preferably, theratio of carbon atoms to transition metal atoms is 50:1 or less, or40:1, 30:1, 20:1, 15:1 or 10:1 or less.

The carbon source is not particularly limited. For example, the carbonsource may be a C₁-C₂₀ hydrocarbon, e.g. a C₁-C₁₀ or C₁-C₅ hydrocarbon,including alkanes, alkenes and alkynes. For example, ethane andacetylene are suitable. Alternatively, the carbon source may be a C₁-C₂₀alcohol, e.g. a C₁-C₁₀ or C₁-C₅ alcohol, such as a monohydroxy alcohol.Typical carbon sources include methane and ethanol. It will beunderstood that mixtures of carbon sources may be employed.

It will be understood that the carbon nanotubes may be produced bychemical vapour deposition, for example thermal chemical vapourdeposition or plasma chemical vapour deposition.

The method may further comprise densifying the carbon nanotubes toproduce a carbon material. For example, the densification may comprisedrawing the carbon nanotubes to form the carbon material. Alternativelyor additionally, the densification may include supplying a densificationagent to the carbon nanotubes. Suitable densification agents includeacetone and divinyl benzene.

Suitable densification methods are described in WO2008/132467 and inReference 1, which are each incorporated herein by reference in theirentirety.

The method may further comprise the step of forming a fibre or film ofcarbon nanotubes. For example, the method may further comprise the stepof drawing the carbon nanotubes into a fibre. The method may furthercomprise the step of forming a yarn from such fibres.

The method may further comprise the step of forming a carbon nanotubepowder, for example by crushing, chopping, cutting or otherwiseprocessing the carbon nanotubes produced.

It will be understood that the carbon material produced by the method,e.g. the fibre, film or yarn, or the carbon nanotube powder produced bythe method, may advantageously have one or more of the optional andpreferred features described above with reference to the carbonmaterials and carbon nanotube powders of the present invention.

Similarly, it will be understood that the method of the presentinvention preferably produces a population of carbon nanotubes havingthe diameter, chirality and metallic properties described above withrespect to the carbon materials and carbon nanotube powders.

The method may further comprise removing some or all of the residualcatalyst particles from the carbon nanotubes.

Preferably the method is performed substantially continuously for atleast 10 minutes, for example for at least 30 minutes, at least 1 houror at least 5 hours.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described, withreference to the accompanying drawings in which:

FIG. 1 shows a Kataura plot.

FIGS. 2A and 2B show a schematic illustration of typical diffractionpatterns obtained for (2A) armchair carbon nanotubes, and (2B) zigzagcarbon nanotubes.

FIGS. 3A, 3B, and 3C illustrate an embodiment of the method of thepresent invention, showing (3A) a schematic illustration of theapparatus used, (3B) a typical temperature gradient within the reactionchamber, and (3C) a schematic flow chart illustration of the method.

FIGS. 4A, 4B, and 4C show SEM images of carbon materials produced in theexamples.

FIGS. 5A, 5B, 5C, and 5D show typical Raman spectra of carbon materialsproduced in the examples.

FIGS. 6A, 6B, and 6C show in (6A) and (6B) extracts from a typical Ramanspectrum of a carbon material produced in the examples, and (6C) anannotated Kataura plot.

FIGS. 7A, 7B, 7C, and 7D show TEM images of carbon materials produced inthe examples.

FIGS. 8A and 8B show the carbon nanotube diameter distribution of carbonmaterials produced in the examples, determined using TEM.

FIGS. 9A and 9B show in (9A) a HREM image of catalyst particleswithdrawn from the reactor in the examples, and (9B) the diameterdistribution of these particles.

FIGS. 10A and 10B show the thermal degradation temperatures of reactantsused in the examples, and relates them to the temperature profile in thereactor.

FIGS. 11A and 11B show in (11A) an example electron diffraction patternobtained from a bundle of carbon nanotubes having armchair chirality,and (11B) a marked up version of the pattern of FIG. 11A with oval marksindicating the location of the diffraction spots.

FIGS. 12A and 12B show in (12A) an example electron diffraction patternobtained from a bundle of carbon nanotubes having armchair chirality,and (12B) a marked up version of the pattern of FIG. 12A with oval marksindicating the location of the diffraction spots.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the method of the present invention will nowbe described. A schematic flow chart illustration of the method is shownin FIG. 3C, and a schematic illustration of the apparatus used is shownin FIG. 3A.

A gaseous mixture of carbon source (e.g. methane), catalyst sourcesubstance (e.g. ferrocene) and arresting agent source substance (e.g.carbon disulphide) is fed into a furnace, carried in a stream of gas(e.g. hydrogen and/or helium). The gas mixture flows through the furnacein a flow direction.

The temperature increases along the flow direction, so that the mixtureis first subjected to a first onset temperature, at which temperaturethe catalyst source substance degrades to initiate growth of catalystparticles. For example, iron atoms may be released, to form catalystparticles comprising iron. Further along the flow direction, the mixtureis subjected to a second onset temperature, at which temperature thearresting agent source substance degrades. The arresting agent is thusreleased, and acts to arrest the growth of the catalyst nanoparticles.The mixture is then subjected to a carbon nanotube formationtemperature, and carbon nanotubes are produced.

As illustrated in FIG. 3A, the resulting carbon nanotubes may bedensified by supplying a densification agent (e.g. acetone). The carbonnanotubes may be drawn into a fibre. A typical winding rate is from 10 ms⁻¹ to 20 m s⁻¹. It will be understood that much higher winding ratesmay be employed.

A typical temperature gradient within the furnace is illustrated in FIG.3B.

Examples

Continuous production of carbon nanotube fibres (schematic as shown inFIG. 3A) was carried out in a vertical ceramic reactor (d=80 mm, I=2 m),with a temperature profile as shown in FIG. 3B. The feedstock containeda carbon precursor (methane) and vapours of a catalyst source substance(ferrocene) and a sulphur source substance, carried by helium. Thefeedstock was introduced in to the reactor through a steel injector tube(d=12 mm, I=90 mm).

On thermolysis of the feedstock components in a reductive atmosphere ofhydrogen followed by synthesis of nanotubes, a plume composed ofentangled nanotubes was obtained which was continuously drawn at 20 mmin⁻¹ and densified with an acetone spray into a fibre. The fibre had atypical diameter of 10 μm.

The effect of two different sulphur precursors, thiophene and carbondisulphide (CDS), on the morphology of the carbon nanotubes constitutingthe fibres produced was investigated. The input concentrations of thevarious precursors were optimised experimentally to provide continuousspinning of the fibre. The elemental ratios used are presented in Table1.

TABLE 1 Input precursor concentrations and the elemental ratiosPrecursors mol min⁻¹ Elemental ratio (×10⁻⁵) (×10⁻³) Carbon CatalystPromoter Fe/S Fe/C Case 1 170 0.21 C₄H₄S 2.5 80 8 Case 2 170 0.21 CS₂ 186 8

(The higher input concentration of sulphur where CDS is used, comparedto where thiophene is used, reflects the fact that smaller catalystparticles are formed where CDS is used (see below); the surface area tovolume ratio of these catalyst particles is higher. Additionally, theCDS becomes available at an earlier stage of catalyst particle growth(again, see below), at which stage there is a higher number density offorming catalyst particles.)

The analyses of the fibre microstructure and the constituting nantotubeswere carried out by electron microscopy (FEI Tecnai F20-G2 FEGTEM, JEOL2000FX and JEOL 6340 FEG), Raman Spectroscopy using a RenishawRamanscope 1000 system (incident light of wavelength 633 nm and 514 nm;acquisition time=10 s; laser spot size=1 μm). The mechanical propertiesof the fibres were investigated with tensile tests using TextechnoFavimat, a dedicated fibre testing equipment which employs a load cellwith a force and displacement measurement range of 0-2 N(resolution=0.0001 cN) and 0-100 mm (resolution=0.1 micrometer)respectively. Testing was carried out at a standard gauge length of 20mm and a test-speed of 2 mm min⁻¹ to acquire the specific strength andspecific stiffness (expressed in N Tex⁻¹, these values are numericallyequivalent to GPa SG⁻¹) of the fibres.

Results

Fibre Composition, Microstructure and Nanostructure

SEM Analysis

Typical SEM images of the condensed and the internal structure of theuncondensed fibres from both the CDS and thiophene runs are presented inFIG. 4. FIG. 4A shows a typical condensed fibre, FIGS. 4B and 4C showsthe internal structure of the fibre prior to acetone densification,where CDS is used as the sulphur precursor (B), and thiophene is used asthe sulphur precursor (C). The nanotubes shown are orientated in thefibre direction. It can also be seen that the CDS fibre shows minimalpresence of extraneous materials (which are generally by-products ofmost CVD processes) in comparison to the thiophene fibre.

Raman Spectroscopy

Raman spectra were acquired on the fibre samples with the polarisationof the incident light parallel to the fibre axis. At least 10 spectrawere collected along the length of the fibre per fibre sample. The fibresamples were 1 cm in length, and acquisitions were spaced at equalintervals along the length. At least five samples obtained from eachsulphur precursor were examined. The typical spectra for fibres obtainedusing CDS and thiophene are presented in FIG. 5. FIG. 5A shows a typicalspectrum for a fibre obtained using CDS, and FIG. 5B shows a typicalspectrum for a fibre obtained using thiophene. FIG. 5C shows the M andiTOLA regions of a typical Raman spectrum for a fibre obtained usingCDS, and FIG. 5D shows the IFM region of a typical Raman spectrum for afibre obtained using CDS.

The positions of the peaks are in the spectra of FIGS. 5A and 5B and theD/G ratios (indicative of fibre purity and crystallinity of thenanotubes) are presented in Table 2.

TABLE 2 The list of positions of the major peaks in the Raman spectraPosition cm⁻¹ Fibre RBM D G G′ I_(D)/I_(G) CDS 194.5 ± 3.8 1320.9 ± 1.11589.7 ± 0.3 2627.2 ± 1.9 0.010 ± 0.003 Thiophene Absent 1331.3 ± 1.51583.8 ± 1.8 2656.4 ± 2.5  0.3 ± 0.04

The low D/G ratios shown by the fibres obtained using CDS in comparisonto those obtained using thiophene are in agreement with the SEM results.They suggest minimal presence of extraneous materials and low density ofdefects in the nanotubes produced using CDS. The distinctive intense lowfrequency ring breathing modes (RBMs) occurring in the spectra from theCDS fibres indicate the presence of single-walled carbon nanotubes. Inaddition, the upshifted G peak (to 1590 cm⁻¹), the downshifted D peak(1320 cm⁻¹), the presence of M (1750 cm⁻¹), i-TOLA (1950 cm⁻¹) andintermediate frequency vibration modes (IFM modes 600-1200 cm⁻¹) confirmthat the fibres obtained with CDS as the sulphur precursor are composedof mainly single-walled carbon nanotubes. All these vibrational featuresare completely absent in fibres obtained with thiophene as the sulphurprecursor and the G band and D band position occurring at 1582 cm⁻¹ and1331 cm⁻¹ are suggestive of the presence of carbon nanotubes with morethan one wall.

G Band and RBM Analysis of CDS Fibres

Further analysis of the G band (FIG. 6A) reveals an internal structureand in addition to the G+ feature at 1590 cm⁻¹ (Lorentzian fit), the G−band occurs as a broad feature at 1552 cm⁻¹ fit with theBreit-Wigner-Fanoline shape which indicates the predominant presence ofmetallic nanotubes. The Fano line shape is given by:

${I(\omega)} = {I_{0}\frac{\left\lbrack {1 + {\left( {\omega - \omega_{BWF}} \right)\text{/}q\; \Gamma}} \right\rbrack^{2}}{1 + \left\lbrack {\left( {\omega - \omega_{BWF}} \right)\text{/}\Gamma} \right\rbrack^{2}}}$

where I₀, ω₀, Γ and q are intensity, normalised frequency, broadeningparameter and line shape parameter respectively.

(FIG. 6A shows the internal structure of the G band, with the LorentzianG+ and the G− exhibiting the Fano lineshape (see the above equation)with fit parameters I₀, ω₀, Γ and q=2256, 1556, 49.5 and −0.20respectively.)

The position of the radial breathing modes (RBMs) can be utilised toobtain the diameters of the nanotubes, as described above. It wasobserved that all the RBM frequencies noticed in the CDS fibre occuraround 200 cm⁻¹ at the excitation wavelength of 633 nm (FIG. 6B)corresponding to the diameter range of 1.2±0.2 nm (d=239/ω_(RBM)). Thiscan be mapped to the Kataura plot, which is a theoretical model thatrelates the diameter of the nanotubes to the optical transitionenergies. Nanotubes of the same diameter can be either metallic orsemiconducting, and the difference in the behaviour is shown in thedifferences in their optical transition energies. From the Kataura plot(FIG. 1 and FIG. 6C) it can be inferred that nanotubes in the diameterrange of 1.1-1.4 nm with optical transition energies in the range of1.96±0.1 eV are metallic, while those with transition energies in therange of 2.41±0.1 eV are semiconducting (the energy range of 0.1 eVtakes in to account any transition energy shifts caused due toenvironmental effects such as nanotube bundling).

Only those tubes with optical transition energies that are in resonancewith the excitation energy (in the case of Raman spectroscopy, theincident laser light) will yield an RBM. While intense RBMs could beobtained with incident light of 633 nm (E_(excitation)=1.96 eV), noresonance, and hence no RBMs, was observed when an incident light of 514nm (E_(excitation)=2.41 eV) was used (FIGS. 4B and 4C). This furtherconfirms that the single-walled carbon nanotubes that constitute the CDSfibre are metallic.

(FIG. 6B shows the representative RBM region of a typical Raman spectrumfor a fibre obtained using CDS. It has a peak at 195 cm¹ withλ_(excitation)=633 nm and the absence of the RBM peak withλ_(excitation)=514 nm. FIG. 6C shows the metallic and semiconductingwindow in the Kataura plot (non-filled circles=metallic nanotubes,filled circles=semiconducting nanotubes) are marked red and greenrespectively on the original colour version of this drawing fornanotubes in the diameter range of 1.1 to 1.4 nm in correlation to theexcitation energies used to acquire the Raman spectra (greenregion=2.41±0.1 eV, 514 nm; red region=1.96±0.1 eV, 633 nm).)

TEM and Electron Diffraction

Analysis by transmission electron microscopy indicates that the bundlesthat constitute the fibres, from both CDS and thiophene, are typicallyin the diameter range of 30-60 nm (FIGS. 7A and 7B respectively). FromHREM analysis, the CDS fibres are composed of SWCNTs and those obtainedwith thiophene as sulphur precursor are composed of collapsed DWCNTs(FIGS. 7C and 7D respectively), confirming the findings from Ramanspectroscopic analysis.

The diameters and diameter distribution of the nanotubes are presentedin Table 3 and FIGS. 8A and 8B. It can be seen that the diametersobtained from TEM analysis of the CDS fibres are in close agreement withthose obtained from Raman spectroscopy (bulk characterisation).

TABLE 3 Average diameters of the single-walled carbon nanotubes obtainedusing CDS, and collapsed double-walled carbon nanotubes obtained usingthiophene, from TEM and RBM. Diameter_(TEM) Diameter_(RBM) Fibre (nm)(nm) CDS: Metallic SWCNT 1.4 ± 0.3 1.2 ± 0.2 Thiophene: DWCNT 7.6 ± 2.3N/A

The diameter distributions, determined using TEM are presented in FIGS.8A and 8B, which shows (8A) the diameter distribution of carbonnanotubes obtained using CDS, and (8B) the diameter distribution ofcarbon nanotubes obtained using thiophene.

Electron diffraction was carried out on fibre bundles (e.g. thoserepresented in FIGS. 7A and B). The electron pattern from fibresobtained using CDS showed a pattern of clear spots, positioned toindicate armchair (n,n) tubes, with a chiral angle of 30°. Incorrelation with the diameter measurements, this suggests that the tubesare (10,10) tubes. Armchair tubes are metallic and hence, these resultsare in agreement with the characterisation by Raman spectroscopy. Theelectron diffraction patterns from the fibres obtained from thiopheneare composed of continuous rings corresponding to (10-10) and (11-20)reflections, which shows that the nanotubes have a continuousdistribution of helicities (i.e. there is a mixture of differentchiralities).

An example electron diffraction pattern is shown in FIG. 11A, for abundle of carbon nanotubes having armchair chirality. Only half of thehexagonal pattern of spots is shown, the remaining spots are obscured bya shade. The arrow indicates the principal axis of the carbon nanotubes.FIG. 11B shows the same image, which has been marked up to show thelocation of the diffraction spots. The position of the three visiblediffraction spots of the hexagonal pattern is indicated with white ovalmarker points. A similar example electron diffraction pattern is shownin FIG. 12A. FIG. 12B shows the same image as FIG. 12A, marked up tohighlight the location of the diffraction spots.

Catalyst Particles

The catalyst particles formed when CDS is used were examined. Theparticles were frozen and withdrawn from the zone of the reactor wherethey form (in the temperature range 400-600C. A HREM image of thewithdrawn catalyst particles is shown in FIG. 9A. The diameterdistribution of these particles, determined using HREM is shown in FIG.9B. This figure shows that the catalyst particles have a narrow sizedistribution.

The average diameter values of the ‘frozen’ catalyst particles is2.5±0.8 nm and the ratio of the average diameter of the catalystparticles to that of the single-walled carbon nanotubes is about 1.8,which is in close agreement with that reported in the literature.

Fibre Properties

Mechanical and Electrical Properties

The mechanical properties of the metallic single-walled carbon nanotubefibre (obtained using CDS) and the double-walled carbon nanotube fibre(obtained using thiophene) are presented in Table 4. The fibres composedof collapsed DWCNT fibres are expected to be superior mechanically, dueto the large contact area between the nanotubes held by van der Waalsforces within the bundles, which is evinced in the tensile strength andstiffness values.

TABLE 4 Fibre characteristics and mechanical attributes properties ofthe metallic SWCNT and DWCNT fibres along with those of copper wire ofelectrical wiring grade. Fibre characteristics Mechanical propertiesLinear Sp. Sp. Diameter density Strength Stiffness Material (μm) (gKm⁻¹) (GPa SG⁻¹) (GPa SG⁻¹) Metallic SWCNT 10-15 0.04 0.5 10 fibre DWCNTfibre 10-15 0.04 1 20 Copper AWG 10 1820 2.3 × 10⁴ 0.03 14

Table 5 below illustrates typical properties of materials, including anon-optimised fibre within the scope of the present invention (finalrow).

Volumetric Linear Specific Current Specific Specific Conductivitydensity density conductivity density Strength Stiffness Material S/m ×10⁶ g/m³ × 10⁶ g/km S/m/g/m³ (A/mm²) GPa/SG GPa/SG Copper 58 8.9 — 6.52-10 0.025 13 (electrolytic) Aluminium 38 2.7 — 14.1  4 0.026 26Steel/Iron 10 7.9 — 1.3 — 0.038 27 Carbon fibre 0.06 1.8 — 0.03 — 1.96128 T300 TORAY High 0.14 1.9 — 0.07 — 2.06 309 performance carbonfibre(M60J) TORAY CNT yarn 0.1-0.7 0.8-1.2 0.02-0.1 0.1-0.7 30 0.8-1.2 60-140“non (depending on metallic” winding rate) CNT yarn 0.7-3 (so far)0.8-1.2 0.02-0.1 0.7-3   80 0.8-1   60-140 “metallic” (depending onwinding rate)

In this table, specific conductivity parameter which takes into accountboth electrical conductivity and the density of a conductor. In Table 5below, the values for specific conductivity were estimated, fromexperimentally obtained values for conductance, length and lineardensity, using the equation below:

$\sigma^{l} = {\frac{G*L}{LD}10^{3}}$

wherein G is conductance (in Siemens), L is length (in metres), and LDis linear density (in tex, or g km⁻¹), and 6′ is specific conductivityin:

$\frac{S\mspace{14mu} m^{- 1}}{g\mspace{14mu} {cm}^{- 3}}$

It will be understood that both the electrical conductivity and thedensity of a conductor are important in many engineering applications,for example in overhead power lines.

In the above examples, without wishing to be bound by theory, it isbelieved that the thermal degradation of ferrocene in hydrogenatmosphere begins at about 673K to yield iron atoms (d=0.3 nm) whichsubsequently grow into nanoparticles (which act as catalysts for thenucleation and growth of nanotubes). These nanoparticles are believed topass through the reactor in the flow direction, along the temperatureprofile. The thermal degradation of the sulphur precursor (CDS orthiophene) in the reaction feedstock leads to the interaction of theiron nanoparticles with sulphur. We call this sulphudisation. Theaddition of sulphur, a recognised promoter in carbon nanotube growth,allows the production of long carbon nanotubes (typically mm). It isbelieved that this can enhance the mechanical integrity of the mass ofcarbon nanotubes produced. This can facilitate the production of carbonmaterials, such as fibres and films, from the carbon nanotubes.

The above examples probe the effect of different sulphur precursors,with varied thermal degradation behaviour. This alters when the sulphurbecomes available to the growing iron nanoparticles. Sulphur is believedto act as an arresting agent, stopping or slowing the nanoparticlegrowth. This is believed to affect the structure of the carbon nanotubesformed.

The thermal stability of CDS is lower than thiophene, especially in areductive hydrogen atmosphere. The adjacent double bonds in CDS areexpected to readily undergo hydrogenation followed by elimination ofsulphur in the form of H₂S. This compound readily sulphudises the ironnanoparticles. Thiophene on the other hand is resistive tohydrogenolysis owing to its stability as an aromatic compound. Where CDSis used, the temperature at which sulphur becomes available to thecatalyst particles is lower than the temperature at which sulphurbecomes available to the catalyst particles when thiophene is used.

As shown in FIG. 10A, the thermal degradation temperatures of ferroceneand CDS are close to each other. Therefore, it is believed that thecatalyst particles are sulphudised in the early stages of their growth.This is believed to result in catalyst particles wherein at least 70% bynumber of the catalyst particles have a diameter in the range from 0.5nm to 4.5 nm. It is believed that the small catalyst particles tend toprovide single-walled carbon nanotubes.

In contrast, there is a much larger difference between the thermaldegradation temperatures of thiophene and ferrocene, as shown in FIG.10B. Therefore, the nanoparticles grow for much longer before theyencounter sulphur obtained by degradation of thiophene. Before this, theiron particles grow to 8-10 nm in diameter. These larger nanoparticlestend to produce larger diameter carbon nanotubes, which tend to bedouble-walled and collapsed.

The experiments were repeated with ethanol as the carbon source (ethanoldecomposition yields a carbon supply at much lower temperatures (about873K) than methane). In the case of carbon nanotube fibres obtained fromethanol, CDS lead to the formation of metallic single-walled carbonnanotubes, while thiophene yielded collapsed double-walled carbonnanotubes. In addition, the effect of the presence of helium (recentlyreported to play a role in the formation of metallic nanotubes;Reference 3) on the formation of metallic nanotubes was tested, bycarrying out the ethanol runs in the absence and presence of helium.Both yielded identical results and the presence of helium did not seemto significantly affect the process.

The preferred embodiments have been described by way of example only.Modifications to these embodiments, further embodiments andmodifications thereof will be apparent to the skilled person and as suchare within the scope of the present invention.

REFERENCES

The content of each of the following references is incorporated hereinin its entirety.

-   1. Koziol, K. et al; High-Performance Carbon Nanotube Fiber; Science    318, 1892 (2007)-   2. Motta, M. S. et al; The Role of Sulphur in the Synthesis of    Carbon Nanotubes by Chemical Vapour Deposition at High    Temperatures; J. Nanosci. Nanotech. 8 1-8 (2008)-   3. Harutyunyan et al; Preferential Growth of Single-Walled Carbon    Nanotubes with Metallic Conductivity; Science 326, 116 (2009)-   4. Carbon Nanotubes; Ed: Jorio, Dresselhaus and Dresselhaus Springer    Verlag Heidelberg 2008-   Kataura et al; Optical Properties of Single-Wall Carbon Nanotubes;    Syn. Met. 103 2555-2558 (1999)

What is claimed is:
 1. A carbon fiber comprising at least 75% by weightof carbon nanotubes, wherein at least 70% by number of the carbonnanotubes have a diameter in the range from 1 nm to 2.5 nm, and whereinthe carbon fiber has a conductivity of at least 0.7×10⁶Sm⁻¹ in at leastone direction. 2-3. (canceled)
 4. A carbon fiber according to claim 1which has a length greater than 0.5 m.
 5. A carbon fiber according toclaim 1, wherein the fiber has a density of at least 0.1 g cm⁻³.
 6. Acarbon fiber according to claim 1, wherein the fiber has a specificstrength of at least 0.1 GPa SG⁻¹ in at least one direction.
 7. A carbonfiber according to claim 1, wherein the fiber has a specific stiffnessof 30 GPa SG⁻¹ or more.
 8. A carbon material according to claim 1,having a plurality of catalyst particles dispersed in the fiber, whereinthe fiber comprises 20 wt % or less of catalyst particles.
 9. A carbonfiber according to claim 8 wherein at least 70% by number of thecatalyst particles have a diameter in the range from 0.5 nm to 4.5 nm.10. A carbon fiber according to claim 1, comprising a plurality ofcarbon nanotubes; a plurality of catalyst particles dispersed in thefiber; and incidental impurities.
 11. A carbon fiber according to claim1, wherein at least 50% by number of the carbon nanotubes aresingle-walled armchair carbon nanotubes.
 12. A carbon fiber according toclaim 8 wherein the catalyst particles comprise transition metal atoms.13. A carbon fiber according to claim 8 wherein the catalyst particlescomprise sulphur atoms.
 14. A carbon fiber according to claim 8 whereinthe catalyst particles comprise an inner core of transition metal atomssurrounded by a shell or cage of sulphur atoms.
 15. A carbon fiberaccording to claim 1 having a diameter equal to or greater than 1 μm andequal to or less than 10 cm.
 16. A carbon fiber according to claim 1wherein a high proportion of the carbon nanotubes are metallic.
 17. Acarbon fiber according to claim 1 wherein a high proportion of thecarbon nanotubes have armchair chirality.
 18. A carbon film comprisingat least 75% by weight of carbon nanotubes, wherein at least 70% bynumber of the carbon nanotubes have a diameter in the range from 1 nm to2.5 nm, and wherein the carbon film has a conductivity of at least0.7×10⁶Sm⁻¹ in at least one direction.
 19. A carbon material comprisingat least 75% by weight of carbon nanotubes, wherein at least 70% bynumber of the carbon nanotubes have a diameter in the range from 1 nm to2.5 nm, and wherein the carbon material has a conductivity of at least0.7×10⁶ Sm⁻¹ in at least one direction, wherein a plurality of catalystparticles are dispersed in the material, wherein the material comprises20 wt % or less of catalyst particles, wherein the catalyst particlescomprise sulphur atoms.